1. Field of the Invention
The present invention relates to a focus error detecting apparatus in which a multi-divided light receiving element detects a far field image reflected by a pit preformed on the recording surface of an optical disk so as to detect a focus error signal by comparing the output of each divided section and a phase comparing apparatus for executing the result obtained by the focus error detecting apparatus.
2. Description of the Related Arts
In recent years, much attention is paid to a focus error detecting apparatus which without using a special optical element, detects a focus error signal based on a far field pattern formed by a laser beam reflected by an optical disk. Various improved techniques have been developed since the technique concerning the focus error detecting apparatus was disclosed in Japanese Laid-Open Patent Publication No. 52-93223. Above all, inventions intended to improve S/N and expand the dynamic range in detecting a focus error have been conspicuously made.
An example of a conventional focus error detecting apparatus is described below with reference to FIG. 5.
FIG. 5 is a block diagram showing a conventional focus error detecting apparatus. The apparatus comprises a light source 20 for emitting laser beams, an optical disk 30, an objective lens 40, a light receiving element 50, buffer amplifiers 70 and 80, pulse generators 90 and 100, sample holding circuits 110 and 120, and a differential amplifier 130. There is recorded information as optically readable pits, i.e. information pits preformed on the recording surface of the optical disk 30 which is rotating. The light receiving element 50 has semicircular light receiving sections 50a and 50b. Lights which have diffracted and scattered at the information pit of the optical disk 30 are incident on the light receiving sections 50a and 50b. Upon receipt of signals outputted from the light receiving sections 50a and 50b, the buffer amplifiers 70 and 80 amplify them up to a predetermined level, thus outputting signals Sa and Sb, respectively. The pulse generators 90 and 100 generates pulse signals Pa and Pb when the signals Sa and Sb cross an appropriate threshold (Vth). The sample-and-hold circuits 110 and 120 sample and hold the signals Sa and Sb at the leading edges of the pulse signals Pa and Pb, respectively, thus outputting signals FE.sup.+ and FE.sup.-. The differential amplifier 130 generates a focus error signal FE based on the difference in voltage between the signals FE.sup.+ and FE.sup.-. This construction is disclosed in, for example, Japanese Laid-Open Patent Publication No. 60-243828.
The operation of the focus error detecting apparatus of the above construction is described below. Supposing that the optical disk 30 is rotating at a constant speed, when the image forming point of laser beams condensed by the objective lens 40 coincides with the information pit recorded on the optical disk 30, namely, at the focal point, the far field pattern of the information pit projected on the light receiving element 50 is approximately uniformly dark. Therefore, at this time, the signals Sa and Sb outputted from each of the light receiving sections 50a and 50b have almost the same phase. When the information pit does not coincide with points other than the focal point of laser beams, i.e., out of the focal point, the far field pattern of the information pit changes like a solar eclipse, i.e., dark portions cross the dividing line of the light receiving element 50. Accordingly, at this time, the signals Sa and Sb have a phase lag or a phase lead. The principle of this phenomenon is omitted herein because it is described in Japanese Laid-Open Patent Publication No. 52-93223. A focus error can be detected by accurately detecting the phase difference.
Referring to FIG. 5, pulse signals Pa and Pb are generated such that the pulse signals Pa and Pb rise when the voltages of the signals Sa and Sb exceed the threshold Vth and fall when the voltages of the signals Sa and Sb become equal to or smaller than those of the threshold Vth. The sample-and-hold circuit 110 samples and holds the signal Sa at the rise of the pulse signal Pb, and the sample-and-hold circuit 120 samples and holds the signal Sb at the rise of the pulse signal Pa. Since the signals Sa and Sb have the same phase at the focal point, the sampled and held signals FE.sup.+ and FE.sup.- have the same value, namely, the threshold Vth. Accordingly, the value of the difference signal FE between the signals FE.sup.+ and FE.sup.- is zero. Out of the focal point, i.e., when the phases of the signals Sa and Sb are different from each other, FE.sup.+ &gt;Vth and FE.sup.- &lt;Vth, or FE.sup.+ &lt;Vth and FE.sup.- &gt;Vth. That is, the difference signal FE is positive or negative. Accordingly, the phase difference between the signals Sa and Sb can be detected as a voltage. The differential amplifier 130 generates a focus error signal based on the voltage.
The above operation is expressed by equations as follows: For simplification, Sa and Sb are expressed as follows: EQU Sa=A sin (.omega.t+.phi./2) (1) EQU Sb=A sin (.omega.t-.phi./2) (2)
Supposing that the pulse signals Pa and Pb rise when the signals Sa and Sb change each from a negative value to a positive value (namely, Vt=0), the timing t.sub.a and t.sub.b of each of the pulse signals Pa and Pb is expressed as follows: EQU t.sub.a =(2n.pi.-.phi./2)/.omega. (3) EQU t.sub.b =(2n.pi.+.phi./2)/.omega. (4)
Substituting equation (4) for equation (1) and equation (3) for equation (2): EQU FE.sup.+ =A sin (2n.pi.+.phi.) (5) EQU FE.sup.- =A sin (2n.pi.-.phi.) (6)
Accordingly, the focus error signal FE is shown as follows: EQU FE=FE.sup.+ -FE.sup.- =2A sin (.phi.) (7)
Thus, it is possible to detect a focus error in the range from -90.degree. to +90.degree.. The focus error signal expressed in equation (7) is shown in FIG. 6.
This art is superior to that disclosed in Japanese Laid-Open Patent Publication No. 52-93223 in that in the latter, the phase difference between the output signals Sa and Sb of each light receiving section is found by the product of a difference signal (Sa-Sb) and a sum signal (Sa+Sb), the phase of which is delayed by 90.degree. while in the former, the phase difference is found by comparing the phases of the signals Sa and Sb with each other. That is, out of the focal point, not only a phase difference is generated between the outputs of the light receiving sections, but also the amplitude of the output of each light receiving section is reduced. That is, "A" of equations (1) through (7) is not a constant but a function of defocus. Moreover, at the time of a defocus, i.e. out of the focal point, the degree of the output reduction of the sum signal (Sa+Sb) is greater than that of the signal Sa or the signal Sb. This is briefly described below. The sum signal is expressed as follows from equations (1) and (2): EQU Sa+Sb=2A cos (.phi./2) sin (.omega.t)
That is, the amplitude of the sum signal is the product of the reduced amount of (A) and cos (.phi./2). For example, when .phi.=90.degree. which is the upper limit of the focus error detection, the amplitude of the sum signal is shown as follows: EQU 2A cos (45.degree. )=1.4A
As shown above, considering the ratio, the amplitude is reduced by 30% compared with that of each signal before it is added to each other. The reduction of the amplitude of the signal in proportion to a focus error amount means the reduction of a focus error detection sensitivity, which substantially means the reduction of the dynamic range of the focus error detection. Therefore, according to the technique disclosed in Japanese Laid-Open Patent Publication No. 60-243828, the dynamic range in focus error detection is widened to some extent as described above.
However, according to the above construction, a phase difference is detected as small as -90.degree. to +90.degree., which is not a sufficient dynamic range to detect a focus error. The normal dynamic range required for a focus error detection system of driving an optical disk is approximately .+-.5 .mu.m. The conventional phase difference detecting range of .+-.90.degree. corresponds to .+-.2 .mu.m in terms of focus error at most. Thus, the conventional focus error detecting apparatus is incapable driving an optical disk to a satisfactory extent.